Multi-channel optical head and data storage system

ABSTRACT

A multichannel optical head and data storage system employs an integrated-optical read channel. The read channel may be fabricated on a planar waveguide structure that separates the data signals of the multiple lasers. The read channel includes focus error sensors and track error sensors. The optical head may be used for amplitude recording systems such as phase-change erasable and write-once media, ablation media, dye-polymer media, and OD-ROM systems.

This application is a division of application Ser. No. 07/926,061, filedAug. 4, 1992, now U.S. Pat. No. 5,353,273.

BACKGROUND OF THE INVENTION

The present invention relates to optical data storage, and moreparticularly, to a data read and servo control system for amulti-channel optical data storage apparatus.

Future optical data storage drives will require increased data rates.One path to higher data rates is to increase the number of lasers thatrecord and read information from the recording surface. If the lasersare focused on independent data tracks, the data rate increases linearlywith the number of the lasers. Going from single-laser systems tomultiple-laser systems complicates the optics, electronics and mechanicsof the head, particularly the read channel. Therefore, an effectivemulti-channel optical head and data read system for a multiple-laseroptical storage device would be desirable. Preferably, the system willbe simple in design, readily manufacturable and easy to maintain andoperate without complicated or costly optics, mechanics or electronics.In this manner, an optical data storage system of increased data ratemay be advantageously realized.

SUMMARY OF THE INVENTION

In accordance with the present invention, a multi-channel laser opticalhead is provided for reading multiple information from an optical datastorage medium having machine readable information encoded thereon in aplurality of data tracks or channels. The optical head is positionableto transmit a plurality of focused laser output beams to the datastorage medium at a focal distance from the optical head. Each laseroutput beam corresponds to a data track or channel on the storagemedium. The storage medium reflects the incident beams as a plurality ofinformation modulated, return beams. The return beams are reflected backto the optical head for processing therein.

The optical head includes a multiple channel laser optical source forgenerating a parallel array of spatially separated, laser output beams.These multiple beams are directed through an optical system within theoptical head and prepared for output to the optical data storage medium.The optical system is formed by a linear array of optical components,preferably including a collimator, a beam shaping optics module, awaveguide structure, a quarter-waveplate and an objective lens.

The collimator is positioned to receive its input from the laser opticalsource and to transmit a plurality of collimated laser output beams. Abeam shaping optics module receives the collimated laser output beamsand produces a plurality of shaped laser output beams. The shaped laseroutput beams pass through the waveguide structure which preferablyallows most of the incident light to pass unhindered. An objective lensis positioned at the output of the optical head. The objective lensreceives the shaped laser output beams and focuses the beams on theoptical data storage medium positioned at the focal distance.

The objective lens receives a plurality of information encoded, returnbeams reflected from the data storage medium. An optical waveguidestructure is positioned to receive the modulated return beamstransmitted through the objective lens to produce a plurality ofredirected, spatially separated signal beams. Optionally, aquarter-waveplate is positioned between the optical waveguide structureand the objective lens to alter the polarization vector of theinformation modulated signal beams relative to the initial unmodulatedlaser source beams, so as to assure high optical efficiency from laserto disk. A plurality of optical detectors are positioned to receive thespatially separated signal beams generated by the optical waveguidestructure and to produce a plurality of output signals representinginformation contained on the optical data storage medium. In a preferredaspect, the optical waveguide structure forms an integrated optical readchannel that redirects and separates the data signals of the multiplelasers. The read channel includes focus error sensors and track errorsensors. As proposed, the optical head may be used for amplituderecording systems such as phase-change erasable and write-once media,ablation media, dye-polymer media, and OD-ROM systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view of a multichannel optical storagehead including a data read channel constructed in accordance with thepresent invention. The head is shown transmitting laser light to andfrom a fragment of an optical storage disk shown in section.

FIG. 1A is a fragmentary plan view of the optical storage disk of FIG. 1showing the recording surface thereof.

FIG. 2 is a detailed diagrammatic side view of a planar waveguidestructure constructed in accordance with the present invention.

FIG. 3 is a diagrammatic top plan view of the planar waveguide structureof FIG. 2.

FIG. 4 is a diagrammatic view of a data and servo detector assembly usedin conjunction with the planar waveguide structure of FIG. 2.

FIG. 5 is a diagrammatic view of an information storage systemconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a multiple laser optical head 2 includes amultiple beam laser source 4 which may be advantageously constructedfrom one or more semiconductor laser devices of conventional design. Forexample, the laser source 4 may include one or more GaAlAs lasersoperating at a laser wavelength about 830 nm. The laser source 4provides a linear array of independent laser channels, only two ofwhich, designated by reference numbers 6 and 8, are illustrated forsimplicity. The laser channels 6 and 8 are spaced to produce respectivedivergent laser beams 10 and 12 having respective beam axes 14 and 16that are angled with respect to each other. The laser beams arepolarized in the plane defined by the axes 14 and 16, as illustrated bythe double-headed arrow designated P in FIG. 1. The arrow P extendsalong the X-axis illustrated as an orientational reference in theFigure.

A lens collimation system 20 collects and collimates the light from thelaser source 4, including the beams 10 and 12 from the laser channels 6and 8. The lens collimator is conventionally formed as a single elementglass lens with at least one aspheric surface using existing glassmolding technology. The collimated light is transmitted through a beamshaping optics system 30. Because the collimated beam cross-sections areelliptical, the beam shaping optics system preferably includes a pair ofprisms oriented to change the elliptical shape of the collimated beamsto a circular cross-sectional shape. The construction of the opticsmodule 30 is conventional in nature. The light is thereafter passedthrough a waveguide read channel system 40 which is inserted into thebeam at a selected angle between the beam shaping optics system 30 and aquarter-wave plate 45. The quarter-wave plate is appropriately orientedto convert the linearly polarized incident light to circularly polarizedlight. The light passing through the quarter-waveplate 45 is focused ona media recording surface 60 by an objective lens 50.

The recording surface 60 is formed as part of an optical storage disk 62which is mounted for rotation about a rotational axis 64, aligned withthe Z-axis shown in FIG. 1. As shown in FIG. 1A, the recording surface60 has a plurality of spaced data tracks or channels 66, 68 and so on,formed thereon. These data channels correspond to the laser beamchannels, e.g., channels 6 and 8, such that each laser beam 10 and 12 isoriented to impinge on a corresponding data channel 66 and 68, and soon. The laser output beams imparted to the recording surface aremodulated in conventional fashion and reflected back into the opticalhead as a plurality of modulated signal return beams. The return beamsreflected from the recording surface 60 are collected by the objectivelens 50 and directed to the read channel system 40, through thequarter-wave plate 45.

The lens 50 is made in the same manner as the collimator 20. By way ofexample, the lens 50 may have a focal length of 3.9 mm and a numericalaperture (NA) of 0.55. The quarter-waveplate is also conventionallyformed from a suitable birefringent material, such as quartz. Inconventional fashion, the quarter-waveplate converts the circularlypolarized light reflected from the disk to linearly polarized lighthaving a polarization direction along the Y-axis shown in FIG. 1. TheY-axis polarized return beams impinge upon the optical waveguidestructure 40.

As illustrated in FIG. 1, the light associated with each return beamtravels at a different angle relative to the Z-axis. This angularseparation of the signal beams is utilized by the read channel system 40to separate the data signals.

Referring now to FIG. 2, the read channel system 40 includes amulti-mode waveguide structure 70 formed on a substrate 72. Thewaveguide 70 and substrate 72 may be fabricated in accordance withwell-known construction methods, the details of which will be apparentto those skilled in the art. For example, the substrate 72 can be madefrom a variety of optically transparent materials, such as glass,saphire, lithium tantalate, lithium niobate thermally oxidized siliconand gallium arsenide (GaAs). These materials are acceptable due to theirability to transmit light energy at wavelengths of interest. Thewaveguide structure 70 is formed as a thin film that is deposited ordiffused on the substrate 72. In a preferred construction, the film maybe sputter coated and thereafter laser annealed to the substrate. Toachieve an effective waveguide, the thickness of the waveguide structure70 should be within an order of magnitude of the wavelengths ofinterest. For example, a waveguide thickness of about 6.5 micron may beused for 830 nm wavelengths transmitted in the transverse electric (TE)mode. The material deposited on the substrate must have an index ofrefraction that is higher than that of the substrate. Waveguidematerials such as polymethylmethacrylate (PMMA), silicon nitride,polymide or Corning 7059 glass film, may all be used provided that asubstrate material having a lower index of refraction is also selected.For example, an effective waveguide structure can be made by sputtercoating a film of Corning 7059 glass film, having a refractive index of1.55, on a quartz substrate having an index of refraction of 1.46.

The read channel system 40 further includes a coupling grating 74 formedon the upper surface of the waveguide 70 (i.e., the rearward face of thewaveguide structure 40 in FIG. 1), preferably by embossing or etching.The coupling grating 74 may be constructed by forming a photoresist maskon the surface of the waveguide 70 with known photolithographictechniques and thereafter ion etching to convert the exposed waveguidesurface into a grating configuration. The coupling grating 74 ispreferably configured to operate as a TE coupler (provided the waveguideis also constructed for the TE modes). Thus, the grating 74 willdiffract the Y-axis polarized return beams incident at discrete anglesinto a plurality of TE modes propagating in the waveguide 72.Alternatively, the coupling grating could be implemented to operate as atransverse magnetic (TM) coupler, provided the waveguide is alsoconstructed for the TM modes and provided further that the lightincident from the laser source 4 is polarized in the direction of theY-axis shown in FIG. 1. For simplicity, the ensuing discussion assumesthat the TE coupling is used.

The coupling grating 74 is advantageously constructed to pass the X-axispolarized light incident from the laser source 4 and to couple theY-axis polarized light reflected from the disk into the waveguide 70. Asshown in FIG. 2, the coupling grating 74 is designed to couple the beams76 and 82 incident at angles Θ₁ and Θ₃ into two TE modes of thewaveguide 70. The light beam 76 is incident on the coupling grating 74at the angle of incidence Θ₁ with respect to an axis 78 extendingparallel to the Z-axis. The Z-axis is illustrated in FIG. 2 as extendingnormal to the X-Y plane of the waveguide structure 70. The light beam 76is diffracted by the coupling grating 74 at the angle Θ₂ and is carriedby the waveguide 70 as a low-order waveguide mode (beam 80). The lightbeam 82 is incident on the coupling grating 74 at the angle of incidenceΘ₃ with respect to the axis 78 and is diffracted at the angle Θ₄ andcarried by the waveguide as a high-order beam 84 illustrated as a brokenline. The angles Θ₁ and Θ₃ are the mode coupling angles of incidence ofthe coupling grating 74 and the waveguide 70. Thus, the coupling grating74 couples TE light traveling at specific mode coupling angles ofincidence into different modes in the waveguide structure 70. (The beampaths shown are only illustrative). The ray angles associated with thewave guide modes are typically quite high, i.e., approaching 90 degreesfor the lowest-order mode. Accordingly, the pitch of the couplinggrating 74 is quite fine, e.g. about 530 nm for wavelengths of about 830nm for a 6.5 micrometer thick Corning 7059 waveguide deposited on aquartz substrate. The angles of incidence are typically low. Forexample, a typical range of angles of incidence would be from about 0.99degrees for the zero-order mode to about 5.27 degrees for higher-ordermodes.

Turning now to FIG. 3, after the light is coupled into the waveguide, achirped grating 86 spatially separates the modes and directs the lightfor detection. In FIG. 3, the waveguide 70 is shown as being ofgenerally rectangular configuration in the X-Y plane. The couplinggrating 74 may be formed as a generally circular region pattern ofparallel gratings oriented in the direction of the short sides of thewaveguide 70. The chirped grating 86 is formed by conventional etchingor the like as a series of parallel gratings oriented at an angle, e.g.,45 degrees, to the orientation of the coupling grating 74 in the X-Yplane. As shown in FIG. 3, the period of the chirped grating 86decreases in the direction of the X-axis as the distance from thecoupling grating 74 increases. The multiple coupled light modestransmitted through the waveguide 70 are illustrated by the arrow A inFIG. 3. These multiple modes are incident on the "chirped" grating 86,which has its grating lines tilted at 45 degrees with respect to thepropagation axis of the waveguide modes. In addition, the period ofgrating 86 varies linearly along its length; hence, the name chirpedgrating. The chirped grating 86 causes each waveguide mode to diffractout perpendicular to the original direction of its propagation at adifferent locations along the grating. The discrete orders of waveguidemodes diffracted out by the chirped grating 86 are illustrated by thearrows B, C, and D in FIG. 3. Each diffracted mode is then focused byits corresponding waveguide lens onto its own detector. Thus, incidentlight modes are deflected at an angle, e.g., 90 degrees, in the X-Yplane. The incident light modes will be reflected at the Bragg matchedgrating pitch at distances from the coupling grating 74, in thedirection of the X-axis, depending on the mode of the light in thewaveguide 70. The grating pitch required to deflect each mode may bereadily determined from the relationship:

    2N.sub.m SinΘ=W/P.sub.m

where,

N_(m) =Mode Index

Θ=Angle of Incidence

W=Wavelength

P_(m) =Grating Pitch

The highest order modes (which have the lowest mode index) will bedeflected by the portion of the chirped grating 86 having the longestperiod. The lowest order modes (which have the highest mode index) willbe deflected by the portion of the chirped grating 86 having theshortest period. Intermediate order modes will be deflected atintermediate period portions of the chirped grating 86. The high order,intermediate order and low order deflected modes are illustrated by therespective arrows B, C and D in FIG. 3.

Data is detected by integrating the energy in each separated mode intoan array 90 of independent data/servo detectors. One or all of thedetectors are segmented to detect focus and tracking error signals. Onepossible geometry for these detectors is shown in FIG. 4. The detectorarray 90 thus includes a plurality of channel structures each includinga waveguide lens 92 and a segmented detector having detector elements94, 96 and 98. The waveguide lens 92 functions as a beam separatorfocusing device that directs and focuses a deflected output mode such asthe mode B from the chirped grating 86 to the detector elements whileexcluding components from other deflected modes, such as the modes C andD. The waveguide lens 92 may be formed in accordance with conventionaltechniques as a buried grating or an integrated optical overlay lensformed directly on the waveguide 70. The light represented by each modechannel is incident, via a waveguide lens, on the detector segments invarying intensities depending on the signal strength of the beamreflected from the media surface 60, the position of the optical head 2relative to the data tracks (tracking error) and the focal distance ofthe objective lens 50 from the media surface 60 (focus error). Thedetector segments 94, 96, and 98 are photosensitive diodes ofconventional design which produce an electrical output signal whoseamplitude is proportional to the intensity of the incident lightthereon. Preferably, the detectors 94, 96 and 98 are integrallyfabricated on the substrate 72.

Using the detectors 94, 96, and 98, a focus signal may be derived from aone-dimensional spot-size measurement (SSM). The focus error signal(FES) is calculated by comparing the output signals from the detectorsegments in accordance with the relationship:

    (94+98)-96

Note the FES is un-normalized because it has been determined thatun-normalized SSM signals exhibit less tracking signal feedthrough thannormalized signals. The tracking error signal is calculated by comparingthe detector output signals in accordance with the relationship:

    (94-98)/(94+98)

Similarly, the data signal is represented by the sum of the signaloutputs of the detector segments:

    (94+96+98).

Segmented servo detectors are preferably placed on each of the detectionchannels. This allows for the generation of skew error signals andsubsequent correction which, in the case of a two beam system, could beaccomplished by rotating the optical head such that both beams areproperly positioned over their respective data tracks. Alternatively,for systems with one master track and multiple slave tracks (i.e.,tracks having no tracking servo datums), one detector could be segmentedand the remaining detectors would have one photosensitive diode.Preferably, the detector array 90 will be fabricated on the substrate 72to provide a fully integrated data read channel. Such fabrication couldbe readily achieved using conventional semiconductor fabricationmethods.

The optical head 2 is preferably incorporated in an optical storagesystem such as the system 100 shown in FIG. 5. The system 100 includes asystem control unit 102 in communication with a host controller (notshown). The system control unit 102 also is in communication with acontrol signal generator 104, a signal processor 106 and atracking/focus control module 108. The optical head 2 is positionedadjacent the disk 62, which is rotatably driven by a motor 110 through aspindle 112. The position of the optical head 2 relative to the disk 62is controlled by the tracking and focusing control module 108. In apreferred embodiment of the optical head 2, all of the elementsillustrated diagrammatically in FIG. 1, except the optical storage disk62, are physically supported in a common frame or housing illustrated asthe rectangle designated "Optical Head" in FIG. 5. Of course, thisallows all components of the optical head to be conveniently movedtogether along the axes E and F.

To read data from the storage medium 62, the host controller sends aread request signal to the system control unit 102. The control unit 102controls the control signal generator 104 to energize the multichannellaser source 4 in the optical head 2. As shown in FIG. 1, the lasersource 4 emits plural laser beams, including beams 10 and 12. The beams10 and 12 are directed through the collimator 20 and the beam shapingoptics 30 to the data read channel 40. The light that is transmittedthrough the read channel 40 and the quarter-waveplate 45 is focused onthe media surface 60 by the focusing element 50. That light is in turnreflected back as a modulated data signal through the focusing lens 50and the quarter-waveplate 45, to the read channel 40.

As previously stated, the spacing of the laser channels 6 and 8 ischosen so the angular separation of the beams 10 and 12 after reflectionfrom the media surface and through the objective lens 50, is equal tothe mode coupling angles of the coupling grating 74 and the multi-modewaveguide 70. The return beams strike the coupling grating 74 in a TEpolarization orientation and a portion of each angular componentincident at a coupling angle is coupled into the waveguide 70. Thecoupled light propagates to the chirped grating 86 in the directionillustrated by the arrow G in FIG. 1, or A in FIG. 3, and then in thedirection shown by arrows B, C and D to the array 90. There, it isspatially separated and focused by the waveguide lens 92 onto the dataand servo detectors 94, 96 and 98, as described above.

The outputs of the detectors 94, 96 and 98 of the array 90 are directedto the signal processor 106 and system control unit 102. Data signalsrepresenting the information on the media surface 62 are transmitted tothe host controller. Tracking error and focusing error signals are sentfrom the system control unit to the tracking/focus control unit 108,which controls the position of the optical head 2 in the direction ofthe double headed arrows E and F. The direction designated by the arrowE represents a tracking control direction, whereas the directiondesignated by the arrow F represents a focusing control direction.

An advantage provided by the data storage system of the presentinvention is the ability to accurately monitor the power output of thelaser source 4 at several locations. For example, if a semiconductorlaser is utilized, laser power can be monitored in the laser itself,i.e., at the rear laser facet. In that case, reflections from the mediasurface 60 are prevented from adversely affecting the power monitoringfunction by the quarter-waveplate 45. The quarter-waveplate rotates thepolarization direction of the return beam by 90 degrees such that thereturn beam is prevented from passing through the TE coupling grating 74and returning to the laser source 4. To achieve this effect, thecoupling efficiency of the grating 74 should be as close to 100 percentas possible.

An alternative power monitoring approach is to allow a portion of theincident laser beams to be diverted by the waveguide structure 40 in thedirection of the arrow H shown in FIG. 1 to a plurality of detectors.This can be achieved by slightly rotating the coupling grating in theX-Y plane such that the power of the detectors will be sin² (Θ) where Θis the rotation angle. Alternatively, a weak transverse magnetic (TM)coupler grating 120 could be placed on the forward face of the waveguidestructure 40. The coupler 120 would couple a portion of the Y-axispolarized incident beams 14 and 16, e.g. 30 percent, into the waveguide70, in the direction of the arrow H. The remaining portion of theincident beams 14 and 16, e.g. 70 percent, would pass through thecoupler 120. TM couplers are known and their implementation in themanner proposed herein need not be described in further detail. Thepower monitor detectors may be integrated on the substrate 72 in thesame manner as the detectors 94, 96 and 98.

When the data storage system of the present invention is used inmagneto-optical recording, wherein information is read by shifts inpolarization direction, it may be further advantageous to employ twoadjacently stacked multi-mode waveguide structures each constructed inthe manner described above with one waveguide operating using the TEmodes of the waveguide and the other using the TM modes. In thisconfiguration, better signal-to-noise characteristics would be expectedas discussed in U.S. Pat. No. 4,868,803.

While a preferred embodiment of a multi-channel optical storage head fora data system has been described, it will be understood thatmodifications and adaptations thereof may occur to persons skilled inthe art. Therefore, the protection afforded the invention should not belimited except in accordance with the scope of the following claims andtheir equivalents.

We claim:
 1. A multichannel beam deflector for deflecting a multichannelbeam having plural nonparallel beam paths from an incident direction toa deflected direction while passing said multichannel beam through saidbeam deflector without deflection in a direction opposite to saidincident direction, said beam deflector comprising a waveguide having aninput coupler grating and a chirped output grating, said input gratingcoupler being positioned to receive the incident multichannel beam andincluding means for coupling the nonparallel beam paths of said incidentbeam at selected angles of incidence into said waveguide as coupledsignal beams in a plurality of coupled modes, said waveguide includingmeans for propagating said coupled signal beams to said output gratingand said output grating including means for spatially separating saidcoupled signal beams for output, and said waveguide further includingmeans for receiving said multichannel beam in a direction opposite tosaid incident direction and allowing said multichannel beam to passthrough said waveguide.
 2. The beam deflector of claim 1 wherein saidwaveguide is formed on a substrate and wherein said input couplergrating is a transverse electric grating mounted on said waveguide at afirst position and said chirped grating is mounted on said waveguide ata second position.
 3. The beam deflector of claim 1 wherein saidwaveguide is formed on a substrate and wherein said input couplergrating is a transverse magnetic grating mounted on said waveguide at afirst position and said chirped grating is mounted on said waveguide ata second position.
 4. A multichannel beam deflector for selectivelydeflecting a multichannel beam having plural nonparallel beam paths froman incident direction to a deflected direction, comprising:means forreceiving a first incident beam in a first direction and allowing it topass transversely therethrough; means for receiving a second incidentbeam representing said first incident beam reflected from a medium as asignal beam in a second direction substantially opposite to said firstdirection: and means for redirecting said signal beam by modalseparation into a plurality of spatially separated output beams in athird direction, said means for redirecting said signal beam including awaveguide oriented to extend in said third direction and having an inputcoupler grating and a chirped output grating, said input coupler gratingbeing positioned to receive said reflected signal beam and to couplesaid reflected signal beam incident at selected angles of incidence intosaid waveguide in a plurality of coupled modes, said waveguide beingconfigured to propagate said coupled reflected signal beam to saidoutput grating, and said output grating being configured to spatiallyseparate said plurality of coupled modes for output.
 5. The beamdeflector of claim 4 wherein said waveguide is formed on a substrate andwherein said input coupler grating is a transverse electric gratingmounted on said waveguide at a first position and said chirped gratingis mounted on said waveguide at a second position.
 6. The beam deflectorof claim 4 wherein said waveguide is formed on a substrate and whereinsaid input coupler grating is a transverse magnetic grating mounted onsaid waveguide at a first position and said chirped grating is mountedon said waveguide at a second position.
 7. The beam deflector of claim 4wherein said waveguide has a thickness within an order of magnitude ofthe wavelengths of said first and second incident beams.
 8. Amultichannel beam deflector for selectively deflecting a multichannelbeam having plural nonparallel beam paths from an incident direction toa deflected direction, comprising a waveguide structure having a couplerinput grating and a chirped output grating configured to (1) receive afirst incident beam in a first direction and allow it to passtransversely through said waveguide, (2) receive a second incident beamrepresenting said first incident beam reflected from a medium as asignal beam in a second direction substantially opposite to said firstdirection, and (3) redirect said signal beam by modal separation into aplurality of spatially separated output beams in a third direction.